Some electric motors having a rotor comprising permanent magnets embedded in the rotor will exhibit a strong reluctance variation, depending on the arrangement of the magnets and barriers. Such motors are often used as electric drive motors having a variable speed, e.g. in electric vehicles or hybrid vehicles and comprise a plurality of rotor poles. The difference between the direct inductance Ld in the direction along the d-axis of a pole and the quadrature inductance Lq along the q-axis of the pole is called saliency and produces a so-called reluctance torque when the drive motor is suitably driven by a switching drive circuit. The d-axis is defined to be aligned with the main flux direction of the rotor, i.e. can be seen as the centre axis of the pole, and the q-axis is defined to be aligned with the quadrature of the d-axis in an electrical perspective, i.e. can be seen as the bisector of two adjacent pole centres. This reluctance torque will act in addition to the torque generated by the magnetic flux of the permanent magnets of the rotor.
Rotors having different layouts of the permanent magnets are known. In some rotors, the magnets are arranged substantially in parallel with the outer circumference of the rotor, i.e. perpendicular to the d-axis. In this configuration, the magnets may be arranged with magnetic layers arranged one above the other. Such an arrangement may be referred to as a parallel magnet arrangement, and may comprise two or more magnetic layers arranged symmetrically relative to the d-axis. Preferably, air pockets are arranged on the opposite ends of the magnets, which are intended to obstruct magnetic flux. In other rotors, the magnets are arranged in a V-shape with one or more magnetic layers arranged symmetrically to the d-axis.
The magnetic saliency in the rotor is the result of the arrangement of the permanent magnets. One or more air pockets are used to direct the magnetic flux in a desired manner. The magnetic layers and the air pockets may be arranged to improve the efficiency of the motor, or to obtain an even flux distribution in order to minimize ripple in the motor.
EP 2494678 describes an electric drive motor that is intended for a vehicle, in particular for a motor vehicle, and which includes a stator and a rotor with at least one pole pair, wherein each pole of a respective pole pair has a magnet arrangement having at least one buried magnetic layer. The drive motor is characterized in that each pole has a number of magnetic flux influencing groups, each of which has a number of air-filled recesses, which are not assigned to a magnet of a respective magnetic layer for purposes of flux conductance. In the shown solution, air filled recesses are arranged between the magnet arrangements, at the outer circumference of the rotor. The purpose of the shown solution is to homogenize the magnetic resistance along the air gap in the rotor iron of the motor, in order to minimize the fluctuations of the magnetic flux density in the teeth of a stator.
This and other similar solutions may solve some problems associated with magnetic saliency and saturation of electric motors, but there is still room for an improved electric motor.